1. Field of the Invention
The present invention relates to filters, and more particularly, to a filter having multiple surface acoustic wave filters connected in parallel.
2. Description of the Related Art
Recently, portable phones and portable information terminals have been widely used due to the development of mobile communication systems. For example, the portable phone terminals use a high frequency range of 800 MHz to 1.0 GHz or 1.5 GHz to 2.0 GHz. Filters employed in the mobile communication systems are required to have a low insertion loss in the pass band and a great attenuation in frequencies outside of the pass band. The transmission filter used in the portable phone terminals is required to have high attenuation in the reception band, and the reception filter used therein is required to have high attenuation in the transmission band. These filters may be formed by surface acoustic wave filters. Japanese Patent Application No. 2003-249842 discloses a surface acoustic wave filter having double-mode filters that are connected in parallel for the purpose of improving the insertion loss in the pass band.
The parallel connection of surface acoustic wave filters reduces the insertion loss in the pass band, but has a difficulty in increasing the attenuation in frequencies outside of the pass band. It may be considered to optimize the film thickness of electrodes of the surface acoustic wave devices connected in parallel to form the filter in order to improve the attenuation. However, the impedance of the filter may deviate from the target value and degrade the insertion loss in the pass band. As described above, it is difficult to realize filters having both a low insertion loss and a high attenuation.